Portable electronic equipment such as laptop computers have storage batteries for powering them when they are used away from a mains supply. The batteries need to be charged, when a mains supply is available, to their maximum so that the maximum computing time is available when the laptop computer is used away from a mains supply.
Care of such batteries is important to ensure a long life and to ensure that they retain a high charge capacity. They should not be overcharged since this can reduce their capacity.
It is possible to monitor the degree of charge for a rechargeable battery by observing the potential of the battery over time as it is charged at a constant current. The potential increases with time up to the point where it becomes fully charged and then starts to reduce. Therefore as soon as the potential reaches a steady value and starts to decline, charging should be stopped. Such an arrangement is described in European Specification No 181112A. 25 A problem arises, however, when recharging batteries in say a laptop computer where the same DC supply generated from the AC mains is used both to power the computer and to provide recharging current. Thus, when the computer is being operated at the same time and the battery is being charged, the output from the charging circuit can be constant but the current drawn by the computer will vary depending upon many factors such as the operation of disc drives. Therefore the remaining current which is available for recharging the batteries will not be constant. Since the potential across the battery varies with the recharging current, one cannot, therefore, avoid the possibility of false conclusions as to whether the battery has reached a maximum potential and so become fully charged if one merely monitors the battery potential without taking into account the charging current.
It is therefore an object of this invention to provide a battery charging circuit where this problem is avoided.